A wide variety of electrically powered heating devices are in existence to provide a wide variety of functions. For example, temperature calibration devices, known as dry-well calibrators, are commonly used in industry to calibrate precision temperature probes.
Conventional dry-well calibrators use Peltier heating/cooling modules generally containing Peltier cells to heat or cool the temperature probes to temperatures that can be set by a user. Electrical current having one polarity is driven through the Peltier cells to cause the temperature of the first substrate to rise relative to the temperature of the second substrate, thereby heating the temperature probe being calibrated. Electrical current having the opposite polarity causes the temperature of the first substrate to fall relative to the temperature of the second substrate, thereby cooling the temperature probe being calibrated. A current is produced by applying an electrical potential across a Peltier cell. A Peltier cell has a specified maximum current. Operating a Peltier cell near or above its maximum current can cause premature performance degradation or failure. Therefore, a limiting factor in the operating range of a dry-well calibrator is the maximum current of the Peltier cells used in the dry-well calibrator. To maximize the operating range of dry-well calibrators, the Peltier cells are frequently driven as close as possible to their maximum current without substantially shortening their lifetime. The current that flows depends on the potential, the resistance of the Peltier cell, and the temperature differential across the Peltier cell. Applying a voltage to a Peltier cell while it has an opposing temperature differential can result in excessive current through the Peltier cell. Specifically, if Peltier cells are still cold when the polarity of power is abruptly switching to induce heating, the cold cells will produce a voltage that effectively increases the heating voltage applied to the cells, thereby potentially inducing excessive current. The same phenomena can occur when quickly transitioning from heating to cooling. It is often necessary to quickly transition dry-well calibrators from heating to cooling, or vice-versa. Sometimes this transition is from full maximum heating to full maximum cooling. During this transition time, until the temperature differentials settle, it is possible for the Peltier cells to temporarily experience excessive current unless means are taken to prevent it.
Peltier cells used in dry-well calibrators are usually stacked on top of each other to provide heating and cooling over a range of temperatures that is wider than this temperature differential of each cell. The total temperature differential of a heating/cooling system is substantially equal to the sum of the temperature differentials that can be developed across all of the stacked Peltier cells. The temperature differential that can be developed between the substrates of each Peltier cell is limited to a specified maximum temperature. Operating a Peltier cell near or above its maximum temperature can cause premature performance degradation or failure. Therefore, a limiting factor in the operating range of a dry-well calibrator is the maximum specified temperature differential of the Peltier cells used in the dry-well calibrator. To maximize the operating range of dry-well calibrators, the Peltier cells are frequently driven as close as possible to their maximum temperature differentials without substantially shortening their lifetime. During a transition from heating to cooling or vice-versa, until the temperature differentials settle, it is possible for some of the Peltier cells in the stack to temporarily experience excessive temperature differentials unless means are taken to prevent it.
Operation of the Peltier cells near or above their maximum specifications can severely limit the useful lives of the cells. Frequent replacement of the Peltier cells can be very expensive, not only because of the cost of the cells, but also because of the cost of labor required to disassemble dry-well calibrators to replace the cells and the downtime costs during such replacement. As a result, there is an inevitable tradeoff between achieving a wide operating range and achieving reliable performance. Even where the Peltier cells are not frequently driven to high maximum temperature differentials, their useful lifetimes can be unreasonably short. It has been recognized that this shortening in the useful life of the Peltier cells can be caused by abruptly changing between heating and cooling functions, particularly between full maximum heating and full maximum cooling. However, despite procedures to simply limit the rate at which the applied potential changes when transitioning from heating to cooling and vice-versa, it is still possible for the maximum current and temperature differential of the Peltier cells to be temporarily exceeded and the useful life of Peltier cells can be unduly limited.
There is therefore a need for a dry-well calibrator using Peltier cells that applies power to the Peltier cells in a manner that does not unduly shorten their useful life.